Interpretive Summary: Leptospirosis is one of the most widespread diseases in the world that is transmitted from animals to humans. Pathogenic bacteria in the genus Leptospira cause leptospirosis, and in humans, Leptospira infection can range in severity from a mild-flu like infection to a severe, acute infection ending in death. Movement of bacteria to new sites of the body is an important component of this disease, yet little is known about how these bacteria establish a niche within different tissues. In this study, tissue from infected animals were examined for the presence of pathogenic Leptospira using fluorescently labeled antibodies to specific bacterial proteins. The distribution of bacteria in different tissues provide new information on how these bacteria disseminate in organs during acute infections and help us understand various aspects of the disease process.

Technical Abstract:
Leptospirosis is a widespread zoonosis caused by invasive spirochetes belonging to the genus Leptospira. Pathogenic leptospires disseminate via the bloodstream to colonize the renal tubules of reservoir hosts. Little is known about leptospiral outer membrane proteins expressed during the dissemination stage of infection. In this study, we describe a novel, surface-exposed lipoprotein, which has been designated LipL45b to distinguish it from a previously described 31-kDa peripheral membrane protein, P31LipL45, that is exported as a 45-kDa lipoprotein. The lipL45b gene encodes a 412-amino-acid polypeptide with a 21-amino-acid signal peptide. Lipid modification of cysteine at the lipoprotein signal peptidase cleavage site, FSISC, is supported by the finding that L. interrogans intrinsically labels LipL45b during incubation in medium containing [14C] palmitate. LipL45b appears to be exported to the leptospiral outer membrane as a 45-kDa lipoprotein, based Triton X-114 solubilization and phase partitioning studies that included the outer and inner membrane controls, LipL32 and LipL31, respectively. Surface immunoprecipitation and whole cell enzyme-linked immunosorbent assay experiments indicate that LipL45b is exposed on the leptospiral surface. Immunohistochemistry studies demonstrate expression of LipL45b by leptospires found in the bloodstream of acutely infected hamsters. Leptospires expressing LipL45b were also found in the intercellular spaces of the liver, within splenic phagocytes, and invading the glomerular hilum of the kidney. Infection-associated expression is supported by the finding that LipL45b is a major antigen recognized by sera from infected hamsters. These findings indicate that LipL45b may be important in the dissemination of leptospires in the mammalian host.